Propulsion systems convert stored energy into mechanical work to drive one or more rotor systems. Energy (e.g., stored in chemical form as fuel) is supplied to an energy conversion device (e.g., a plurality of internal combustion engines such as a turbine engine, spark ignition engine, or compression ignition engine), which converts the energy into mechanical work. A drive system transmits mechanical work through a plurality of transmission mechanisms (e.g., main rotor gearbox(es), a tail rotor gearbox, intermediate gearbox(es), drive shafts, drive couplings, etc.) to drive thrust generating rotors.
In general, a physical tachometer is a device for measuring rotational speed of rotating machinery and generating multiple equally spaced pulses per revolution, both of which are often used for condition monitoring, fault diagnosis, and system reliability improvement of the plurality of transmission mechanisms. The use of physical tachometers presents challenges when used for condition monitoring of complex rotating machinery having many components, which may be spatially distributed, requiring speed changes throughout the system and multiple gearboxes. First, the use of a global tachometer for processing of dynamic or vibration data measured elsewhere in the plurality of transmission mechanisms can lead to system latencies and inaccuracies. Second, the integration of multiple tachometers can lead to expensive solutions with respect to monetary and weight penalty costs. Third, physical tachometer signals can be degraded due to various operational issues such as dirty environment or misalignment caused by incorrect installation or inadvertent misalignment during maintenance events. In addition, the introduction of multiple sensors can lead to overall aircraft reliability issues.